A receiver, also known as User Equipment (UE), mobile station, wireless terminal and/or mobile terminal is enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The communication may be made e.g. between two receivers, between a receiver and a wire connected telephone and/or between a receiver and a server via a Radio Access Network (RAN) and possibly one or more core networks.
The receiver may further be referred to as mobile telephones, cellular telephones, computer tablets or laptops with wireless capability. The UEs in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another receiver or a server.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node, or base station e.g. a Radio Base Station (RBS), which in some networks may be referred to as transmitter, “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used. The radio network nodes may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the radio network node/base station at a base station site. One radio network node, situated on the base station site, may serve one or several cells. The radio network nodes communicate over the air interface operating on radio frequencies with the receivers within range of the respective radio network node.
In some radio access networks, several radio network nodes may be connected, e.g. by landlines or microwave, to a Radio Network Controller (RNC) e.g. in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), radio network nodes, which may be referred to as eNodeBs or eNBs, may be connected to a gateway e.g. a radio access gateway, to one or more core networks.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the radio network node to the receiver. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction i.e. from the receiver to the radio network node.
The downlink of contemporary wireless systems, such as the 3GPP Long Term Evolution (LTE) cellular communication system, is based on Orthogonal Frequency Division Multiplex (OFDM) transmission, which uses time and frequency resource units for transmission. OFDM is a method of encoding digital data on multiple carrier frequencies. OFDM is a Frequency-Division Multiplexing (FDM) scheme used as a digital multi-carrier modulation method. A large number of closely spaced orthogonal sub-carrier signals are used to carry data. The data is divided into several parallel data streams or channels, one for each sub-carrier. The smallest time-frequency resource unit, called resource element (RE), comprises a single complex sinusoid frequency (sub-carrier) in an OFDM symbol. For the purpose of scheduling transmissions to the different receivers/UEs, the resource elements are grouped into larger units called Physical Resource Blocks (PRBs). A PRB occupies a half of a subframe, called “slot”, consisting of six or seven consecutive OFDM symbol intervals in time domain (0.5 millisecond in total), and twelve consecutive sub-carrier frequencies in frequency domain (180 kHz in total). Each PRB is indicated by a unique index: nPRBε[0, NRBDL−1] denoting the position of the sub-band that the PRB occupies within a given bandwidth, where NRBDL−1 is the total number of PRB within the bandwidth. The maximum number of PRBs NRBmax,DL, associated with the largest LTE bandwidth (20 MHz), is 110. The relation between the PRB number nPRB in the frequency domain and resource elements (k,l) in a slot is nPRB=└k/NscRB┘.
The LTE Rel-8/10 defines a Physical Downlink Control Channel (PDCCH) as a signal containing information needed to receive and demodulate the information transmitted from the radio network node/eNodeB to a receiver/UE through the Physical Downlink Shared Channel (PDSCH). PDSCH is performed over 1 ms duration (which is also referred to as a subframe) on one or several RBs and a radio frame consists of 10 subframes.
The PDCCH is transmitted in a control region that may occupy up to three OFDM symbols at the beginning of each subframe, whereas the remaining of the subframe forms the data region used for the transmission of the PDSCH channel.
The LTE Rel-11 supports a new control channel scheduled within the time-frequency resources of the downlink data region. Unlike the legacy LTE downlink common control channel PDCCH, this new feature, known as Enhanced Physical Downlink Control Channel (EPDCCH), has the distinct characteristic of using Demodulation Reference Signals (DMRS) for demodulation and, consequently, the capability to associate each EPDCCH with a specific receiver/UE, as DMRS are receiver-specific.
The EPDCCH structure is fundamentally different from that of the PDCCH, e.g., it is based on UE-specific demodulation reference signals instead of cell-specific reference signals. While the PDCCH is transmitted over the whole system bandwidth, the EPDCCH may be confined to a configurable UE-specific set of RBs (i.e., EPDCCH set) and the receiver/UE may be configured with multiple EPDCCH sets. Each EPDCCH set comprises a group of (e.g., 2, 4 and 8) Physical Resource Block (PRB) pairs and each PRB pair comprises a set of (e.g., 16) Enhanced Resource Element Groups (EREGs). In turn, the set of REGs in a PRB pair constitute Enhanced CCEs (ECCEs). The number of ECCEs per PRB pair may typically be 2 or 4 (i.e., corresponding to 8 and 4 EREGs, respectively), depending on the subframe type, i.e., it may be time-varying. Depending on the radio link conditions, an EPDCCH may be transmitted on a set of ECCEs, e.g., 1, 2, 4, 8, 16 or 32 ECCEs, located either within one or a few PRB pairs (i.e., localized transmission), or on all PRB pairs of the EPDDCH set (i.e., distributed transmission). The ECCEs are enumerated per each EPDCCH set. The EPDCCH also supports Multi-User Multiple Input Multiple Output (MU-MIMO), such that several EPDCCHs could be transmitted on the same set of ECCEs using different antenna ports.
For EPDCCH demodulation, four DMRS antenna ports {7, 8, 9, 10} may be used, as stated in 3GPP TSG Ran WG1, “Final Report of 3GPP TSG RAN WG1 #67 v1.0.0”, R1-120001, February, 2012. In order to reduce the detection complexity, the antenna port used for EPDCCH transmission shall be known to the receiver/UE. One way to indicate the used antenna ports to the receiver/UE is an implicit association between antenna ports and useful ECCEs. Several methods for antenna port associations have been discussed, and the latest agreement is that with localized allocation, each ECCE index is associated by specification with one antenna port, see 3GPP TSG Ran WG1, “Final Report of 3GPP TSG RAN WG1 #69 v0.2.0”, R1-12381 June 2012.
The EPDCCH transmission can be either localized or distributed with the granularity of one PRB pair. With localized transmission, the EPDCCH for a receiver/UE is typically transmitted over a single PRB pair scheduled by the associated radio network node/eNodeB based on CQI feedback information (frequency selective scheduling); with distributed transmission, the EPDCCH is transmitted over multiple PRB pairs to achieve frequency diversity. The latter scheme is useful if there is no feedback or the available feedback is not reliable, although more resources (i.e. PRBs) are locked for EPDCCH transmission.
The EPDCCH design exploits a receiver/UE specifically configured search space. For a given receiver/UE, the serving radio network node (such as e.g., eNodeB in LTE) may configure one or multiple sets of Physical Resource Block (PRB) pairs (EPDCCH sets in LTE terminology) that can be used to transmit the downlink control channel signals to the receiver/UE. Each EPDCCH set can be configured for either localized or distributed EPDCCH transmission. A distributed EPDCCH transmission shall use all PRB pairs within an EPDCCH set, while for localized EPDCCH transmission, the EPDCCH shall be transmitted over one or more PRB pairs within an EPDCCH set. The unit block for EPDCCH multiplexing and blind decoding is the Enhanced Control Channel Element (ECCE), which consists of a block of resource elements in a PRB pair. When EPDCCH is transmitted, one, two or four ECCEs can be aggregated together based on the payload size and coding rate of the transmitted EPDCCH creating aggregation levels of one, two or four, respectively. Therefore, one PRB pair can contain one or more ECCE depending on the ECCE size and the mapping rule used to map EPDCCH to the PRB pair.
A relevant design aspect yet to be specified for the LTE Rel-11 is the receiver's EPDCCH searching procedure within its search space, i.e. within the EPDCCH sets configured for said receiver/UE. Further, the control channel signal can be transmitted either in a distributed or a localized manner. The current LTE design implies that a number of blind decoding attempts shall be configured for EPDCCH detection. The overall number of allowed blind decoding attempts shall then be split among all configured EPDCCH sets for a receiver/UE. How the available decoding attempts shall be split among the EPDCCH sets or the total amount of blind decoding attempts allowed for a receiver/UE are not solved. Further, how multiple EPDCCHs can be multiplexed together within a set, how the total number of allowed blind decoding attempts for a receiver/UE shall be split among different sets of different size (i.e., in terms of number of PRB pairs), and more generally the control channel searching procedure for the receiver/UE, are still open issues.
The issue to be resolved is how to design an EPDCCH transmission scheme that can enable an efficient EPDCCH search that minimizes the number of blind decoding attempts at the receiver/UE while ensuring reliable detection of the control channel signals.
In one solution described in Ericsson, ST-Ericsson, “On Enhanced PDCCH Design”, R1-112928, Zhuhai, China, Oct. 10-14, 2011, the frequency location for the EPDCCH is indicated by a new Downlink Control Information (DCI) format transmitted in the Physical Downlink Control Channel (PDCCH) region. This hierarchical design implies that a receiver/UE first performs blind detection in the PDCCH region to find the new DCI format, and then determines whether there is EPDCCH in the data region according to the status of the new DCI format detection. This solution relies on explicit signaling of scheduled EPDCCH time-frequency resources (via PDCCH), and it does not comply with the latest EPDCCH search-space design.
The current LTE design specifies that a receiver/UE will be configured with a fixed number of allowed blind decoding attempts for searching its control channel signals within its search-space. The total number of allowed blind decoding attempts for a receiver/UE shall be split among different sets of different size (i.e., in terms of number of PRB pairs), and, unless a more specific searching rule is specified, a receiver/UE shall scan all the EPDCCH set configured for its EPDCCH transmission and perform a fixed number of blind decoding attempts in each set, regardless whether an EPDCCH signal is found or not. Although this method can assure the highest detection reliability (if a sufficient number of decoding attempts is performed in each set), it also requires high computational complexity at the receiver/UE and high energy consumption, see e.g., Huawei, HiSilicon, “DMRS sequences for EPDCCH”, R1-120870, Dresden, Germany, 6-10 Feb. 2012, for a thorough analysis.
The alternative method proposed in Huawei, HiSilicon, “DMRS sequences for EPDCCH”, R1-120870, Dresden, Germany, 6-10 Feb. 2012, consists in using an EPDCCH signature sequence to re-modulate the EPDCCH DMRS for all receivers/UEs. Such signature sequence can allow each receiver/UE to identify any PRB pair, within the configured search space, carrying either a single or multiple EPDCCHs (so-called “candidate EPDCCH PRB pairs”, see Huawei, HiSilicon, “Scrambling sequence for EPDCCH detection”, R1-120993, Jeju, Korea, Mar. 26-30, 2012), and then to perform blind decoding within each candidate PRB pair in order to find its own EPDCCH. The results in Huawei, HiSilicon, “DMRS sequences for EPDCCH”, R1-120870, Dresden, Germany, 6-10 Feb. 2012, demonstrate that the number of operations for a single blind EPDCCH detection attempt is much larger than for a single DMRS signature sequence detection attempt. Hence, the average number of operations for an EPDCCH detection scheme based on DMRS signature sequences is dominated by the number of blind EPDCCH detection attempts which are made if the signature sequence detection fails and/or Cyclic Redundancy Check (CRC) fails. This method, however, could not be applied in case of distributed EPDCCH transmission, and does not take into account the latest EPDCCH search space design details.
Hence, it is a general problem to enable an improved method for detecting control channel resources at the receiver/UE.